$base:#efefef;

body{
  background: $base;
}
.mask{
  width: 100%;
  height: 100%;
  background:rgba(0,0,0,0.5);
  position: absolute;
  z-index: 1;
  top: 0px;
  left: 0px;
  display: flex;
  justify-content: center;
  align-items: center;
  .change{
    width: 400px;
    height: 400px;
    background:#fff;
    border-radius: 4px;
  }
}